This is a proposal requesting shared instrument grant funds to establish a precision scanning microdensitometer facility. The equipment in this facility will be used primarily to digitize high resolution electron micrographs and electron diffraction patterns, both of which are recorded on photographic emulsions. A major part of the electron crystallographic work is concerned with the structure and function of cell membrane proteins. Another electron crystallographic application is concerned with the structure analysis of thin crystalline plates of highly polymerized DNA. All of these projects are concerned with molecular structure analysis on basic problems that are currently of interest in the understanding of cell biology and biochemistry. In another type of application, the equipment will be used to digitize the position of heavy ion "tracks" that are produced in thin plastic sheets and developed by solvent-etching. In this case the plastic films are used to record heavy ion radiographs as part of an experimental program in tumor localization and heavy ion radiotherapy.